Identifying Lanthanide Energy Levels in Semiconductor Nanoparticles Enables Tailored Multicolor Emission through Rational Dopant Combinations

ConspectusThe unique photon emission signatures of trivalent lanthanide cations (Ln3+, where Ln = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) enables multicolor emission from semiconductor nanoparticles (NPs) either through doping multiple Ln3+ ions of distinct identities or in combination with other elements for the creation of next-generation light emitting diodes (LEDs), lasers, sensors, imaging probes, and other optoelectronic devices. Although advancements have been made in synthetic strategies to dope Ln3+ in semiconductor NPs, the dopant(s) selection criteria have hinged largely on trial-and-error. This combinatorial approach is often guided by treating NP-dopant(s) energy transfer dynamics through the lens of spectral overlap. Over the past decade, however, we have demonstrated that the spectral outcomes correlate better with the placement of Ln3+ energy levels with respect to the band edges of the semiconductor, and oxide, host.In this Account, we describe how the Ln3+ energy level alignments affect the dopant emission intensities and dictate interdopant energy transfer processes in semiconductor nanoparticle hosts. This Account begins with a concise primer on the emission characteristics of trivalent lanthanides, the challenges that are associated with realizing meaningful lanthanide luminescence, and how semiconductor nanoparticles can act as a host to sensitize lanthanide emission. We then describe a semiempirical approach that can be used to place the lanthanide ground and luminescent energy levels with respect to the band edges of the host semiconductor nanoparticle. The ability of this model to track and predict the lanthanide sensitization efficiency is illustrated for singly doped zinc sulfide (ZnS), titanium dioxide (TiO2), and cesium lead chloride (CsPbCl3) perovskite hosts. Next, we discuss how knowledge of energy level offsets can be used to select dopant(s) for tunable multicolor emission by identifying different charge trapping processes for semiconductors doped with single and multiple lanthanides and discussing their impact on sensitization outcomes. Following this discussion, the Account lists viable Ln3+ combinations in ZnS NPs based on the charge trapping model and shows the limitations of spectral overlap models in predicting viable Ln3+ dopant combinations. Feasible f-f and d-f codopant combinations based on charge trapping are presented for TiO2 and CsPbCl3 NPs. The intricacies of interdopant energy migration and spin considerations that dictate the dopant(s) sensitization efficiencies are made known. Finally, we use these considerations to predict NP-dopant(s) combinations that should exhibit concerted emissions from the blue to the near-infrared (NIR) region, thereby enabling the design of bespoke optoelectronic properties. The Account ends with some forward-looking thoughts, arguing for the need to develop better quantitative models in order to explore the Ln3+ sensitization mechanisms and presenting ideas for applications of doped semiconductor NPs in energy and health that would be aided by interdopant energy transfer dynamics.

Host sensitized lanthanide photoluminescence from post-synthetically modified semiconductor nanoparticles depends on reactant identity

This work investigates the photoluminescence characteristics where cadmium selenide (CdSe) and zinc sulfide (ZnS) nanoparticles are treated post-synthetically by the trivalent lanthanide cations (Ln³⁺) [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] separately to form either CdSe/Ln or ZnS/Ln nanoparticles. Host sensitized Ln³⁺ emission was found to be present only in CdSe/Eu, CdSe/Tb, ZnS/Eu, ZnS/Tb and ZnS/Yb nanoparticles. In all the cases tuning of emission of the nanoparticles has been observed, irrespective of the presence or absence of host sensitization. The elemental compositions of CdSe and ZnS nanoparticles upon post-synthetic treatment show a remarkable difference. Incorporation of lanthanides in the nanoparticles is evident with significant alteration in the anionic content, and complete cation exchange of either Cd²⁺ or Zn²⁺ by Ln³⁺ has not been detected; as evaluated from energy dispersive X-ray spectroscopy. Further evaluation on this comes from considering thermodynamic parameters of inter cation interaction. In cases where the host sensitized Ln³⁺ emission have been observed, luminescence lifetime measurements reveal significant protection of Ln³⁺ in the nanoparticles. Noticeable difference in photophysical properties for a given Ln³⁺ has been realized in the two hosts. The photophysical observations have been rationalized using (i) charge trapping mediated host sensitized dopant emission, (ii) autoionization of excited electrons, and (iii) environment induced photoluminescence quenching. The post-synthetic modification discussed in the present work provides an easy and less synthetically demanding room temperature based protocol to avail lanthanide incorporated (doped) semiconductor nanoparticles that can potentially use the unique emission properties of the lanthanide cations.

Host-sensitized sharp samarium emission from doped titanium dioxide nanoparticles as non-cytotoxic photostable reporters for live-cell imaging

This work introduces APTMS capped Ti(Sm)O₂ nanoparticles as reporters for live-cell imaging with negligible cytotoxicity and resistance to photobleaching.